1. Field of the Invention
The present invention relates to electrostatographic apparatus such as copiers and printers, and more particularly, to apparatus and method, wherein a fixing device for fixing toner images to a substrate is provided for producing high-quality reflection copy images and high-quality projection copy images.
2. Background Art
Electrostatographic process apparatus which, for example, produce or reproduce toned images on selected substrates such as sheets of paper or transparencies by employing electrostatic charges and toner particles on an insulated photoconductive surface, typically operate through a sequence of currently well known steps. These steps include (1) charging of the insulated photoconductive surface with electrostatic charges, (2) forming a latent image electrostatically on such surface by selectively discharging areas on such surface, (3) developing the electrostatic image so formed with fusable powdery marking or toner particles, (4) transferring the toned image to a suitable substrate such as a sheet of paper or a transparency for fixing thereon by a heat and pressure fixing device, for example in order to form a permanent record copy on such a substrate, and (5) cleaning by removing residual toner and/or other particles from the photoconductive surface in preparation for similarly producing another permanent record copy.
The quality of the permanent record copy so fixed on the substrate depends in significant part on the effectiveness of the fixing device or apparatus. For example, to be effective a heat and pressure fixing apparatus should substantially melt and thus cause the dry powdery marking or toner particles which form an image to flow such that there is little or no perceivable scattering of such particles when the image of the copy is viewed. The degree of melting or flow is dependent, for example, on how long heat and pressure are applied to such particles, and also on how the copy image they form is being viewed.
Copy images on transparencies, for example, are viewed differently from those on sheets of paper. Copy images on a sheet of paper are viewed by reflecting light off of the image side of the sheet of paper. Copy images on a transparency sheet, however, are viewed by shining light from the backside of the sheet, through the sheet and past the image side thereof into a lens and then onto a projection surface or screen. As a consequence, any significant scattering or significant amount of incompletely melted toner particles in the projected image on a transparency sheet will show up as an image defect by causing the light to be scattered away from the lens, thus desaturating the projected image. The same degree of scattering or of incompletely melted toner particles on a reflection sheet-of-paper copy, however, ordinarily may not be perceived as significantly detracting from image quality. Consequently, there is, therefore, a need to melt substantially all the toner particles of a projection copy image, but no need to completely heat and melt toner particles forming reflection copy images on a sheet of paper. In fact, attempting to completely fuse images on a sheet of paper can result in overfusing or overheating of the sheet of paper. Such overheating may undesirably result in other types of image copy defects such as wrinkling and blistering of the sheet of paper.
It is, therefore, difficult and ordinarily cumbersome to attempt to interchangeably produce high-quality reflection and projection image copies on sheets of paper and on sheets of transparencies respectively, using the same fixing apparatus in a copier or printer, for example without undesirably slowing down or reducing the fixing apparatus speed for transparency substrates.